The present invention relates generally to X ray radiation imaging systems and more particularly to a method and apparatus for collimating X rays to avoid excess dosage to the patient.
Collimators are used in applications where it is desirable to permit only beams of radiation emanating from the radiation source in a particular direction to pass beyond a selected path or a plane. In radiation imagers, collimators are used to ensure that no radiation beams emanating along a direct path from the radiation source miss the detector and hit unintended parts of the object. Collimators are positioned to substantially absorb the undesired radiation. Collimators are traditionally made of a material that has a relatively high atomic number. Collimator design affects the field of view of the imaging system. With the introduction of new imaging applications, the conventional collimators have a disadvantage that excess X rays can spill past the edge of the detector surface (or other predetermined exposure area), or that not the entire detector surface (or other predetermined exposure area) is exposed to incident X rays.
In the conventional imaging systems, collimators are used for standard examinations. One such configuration of a collimator comprises an X ray opaque metal with a simple aperture. In another collimator embodiment the aperture is formed by blades that are motor driven to fixed opening sizes. During the course of an X ray exam, typical in tomosynthesis, stereotaxy, stereo imaging and mammography where the X ray source travels in a prescribed arc (or other prescribed trajectory) around the object (patient), it is important to prevent any unnecessary X ray dose to reach the object. Presently the limitation of radiation exposure to the object is governed by US regulation CDRH 21 CFR 1020.30(k).
In such advanced imaging systems, it is desirable to minimize the radiation exposure to the patient, minimize the complexity of the collimator in terms of its mechanical, electrical and software implementation, assure high speed of response of the system so that multiple images can be acquired in rapid succession, control the movement of the collimator with respect to other motion in the imaging system, and assure maximum field of view at the detector consistent with system constraints.
Briefly, in accordance with one embodiment of the invertion, a. radiation imaging system comprises a movable radiation source adapted to be disposed in a plurality of respective radiation source positions, a radiation detector and a collimator assembly. The collimator assembly comprises a collimator and a collimator positioning apparatus which is configured to displace the collimator in a plurality of respective collimator positions. Further, each of the collimator positions is coordinated with at least one of the radiation source positions such that a radiation beam emanating from the radiation source is collimated to limit radiation to a predetermined exposure area on the detector.
In accordance with another embodiment of the present invention, a method for radiation imaging comprises positioning a radiation source in a plurality of respective radiation source positions; displacing a collimator in a plurality of respective collimator positions where each of the collimator positions corresponds to a respective one of the radiation source positions such that a radiation beam emanating from the radiation source is collimated to limit the incident radiation to a predetermined exposure area on the detector; and detecting the radiation beam on the radiation detector.
In accordance with another embodiment of the present invention, a radiation imaging system comprises a movable radiation source, a radiation detector and a collimator comprising an adjustable geometry aperture assembly configured such that an adjustment of the aperture geometry is synchronized with the movement of the radiation source and coordinated with the radiation source position so as to limit the incident radiation to a predetermined exposure area at the detector.
In accordance with another embodiment of the present invention, a method for radiation imaging, comprises moving a radiation source in a plurality of radiation source positions; adjusting an aperture by synchronizing the aperture geometry adjustment with the movement of the radiation source and coordinating at least one of the position and the shape of the aperture with the respective position of the radiation source such that a radiation beam emanating from the radiation source is collimated to limit the incident radiation to a predetermined exposure area; and detecting the radiation beam on a radiation detector.